Saturn’s moon Titan is home to seas and lakes filled with liquid
hydrocarbons, but what makes the depressions they lie in? A new study
suggests that the moon’s surface dissolves in a similar process that
creates sinkholes on Earth.

Apart from Earth, Titan is the only body in the Solar System known to
possess surface lakes and seas, as seen by the international Cassini
mission. But at roughly –180°C, the surface of Titan is very cold and
liquid methane and ethane, rather than water, dominate the
‘hydrological’ cycle.

Indeed, methane and ethane-filled topographic depressions are
distinctive features near the moon’s poles. Two forms have been
identified by Cassini. There are vast seas several hundred kilometres
across and up to several hundred metres deep, fed by river-like
dendritic channels. Then there are numerous smaller, shallower lakes,
with rounded edges and steep walls, and generally found in flat areas.
Many empty depressions are also observed.

The lakes are generally not associated with rivers, and are thought fill
up by rainfall and liquids flooding up from underneath. Some of the
lakes fill and dry out again during the 30-year seasonal cycle on Saturn
and Titan.

But quite how the depressions hosting the lakes came about in the first place is poorly understood.

A team of scientists have turned to home for the answer and discovered
that Titan’s lakes are reminiscent of ‘karstic’ landforms seen on Earth.
These are terrestrial landscapes that result from erosion of soluble
rocks such as limestone and gypsum in groundwater and rainfall
percolating through rocks. Over time, this leads to features including
sinkholes and caves under humid climates, and salt-pans under more arid
climates.

The rate of erosion depends on factors such as the chemistry of the
rocks, the rainfall rate and the surface temperature. While all of these
aspects clearly differ between Titan and Earth, the underlying process
may be surprisingly similar.

A team lead by ESA’s Thomas Cornet calculated how long it would take for
patches of Titan’s surface to dissolve to create these features. They
assumed that the surface is covered in solid organic material, and that
the main dissolving agent is liquid hydrocarbons, and took into account
present-day models of Titan’s climate.

The scientists found that it would take around 50 million years to
create a 100 m-deep depression at Titan’s relatively rainy high polar
latitudes, consistent with the youthful age of the moon’s surface.

“We compared the erosion rates of organics in liquid hydrocarbons on
Titan with those of carbonate and evaporite minerals in liquid water on
Earth,” describes Thomas.

“We found that the dissolution process occurs on Titan some 30 times
slower than on Earth due to the longer length of Titan’s year and the
fact it only rains during Titan summer.

“Nevertheless, we believe that dissolution is a major cause of landscape
evolution on Titan, and could be the origin of its lakes.”

In addition, the scientists calculated how long it would take to form
lake depressions at lower latitudes, where the rainfall is reduced. The
much longer timescale of 375 million years is consistent with the
relative absence of depressions in these geographical locations.

“Of course, there are a few uncertainties: the composition of Titan’s
surface is not that well constrained, and neither are the long-term
precipitation patterns, but our calculations are still consistent with
the features we see today on Titan’s relatively youthful
billion-year-old surface,” says Thomas.

“By comparing Titan’s surface features with examples on Earth and
applying a few simple calculations, we have found similar land-shaping
processes that could be operating under very different climate and
chemical regimes,” says Nicolas Altobelli, ESA’s Cassini–Huygens project
scientist.

“This is a great comparative study between our home planet and a dynamic
world more than a billion kilometres away in the outer Solar System.”

More information

“Dissolution on Titan and on Earth: Towards the age of Titan’s karstic
landscapes,” by T. Cornet et al. is accepted for publication in the Journal of Geophysical Research – Planets, doi: 10.1002/2014JE004738

Cassini–Huygens is a cooperative project of NASA, ESA and ASI, the Italian space agency.